What are the likely Soviet responses if President Reagan should decide to stop observing the treaty?

For one, the Kremlin would likely make a similar announcement, stressing that the decision was forced upon it and taken more in sorrow than in anger.

For another, Soviet officials say privately they would seriously review the current set of Geneva arms control talks. What purpose would be served, one official asks, by hammering out new agreements, when Washington abrogates existing ones?

But some Western diplomats say it is highly unlikely that the Kremlin would actually break off the current Geneva negotiations.

``It pays them to stay in there,'' says one, ``and hope that the pressures will build on the [Reagan] administration'' to make some sort of compromise.

Notably, the Soviet news media have made scant reference to the debate in Washington over whether to adhere to the treaty. Nor, some Western diplomats say, has much concern been expressed privately.

One Soviet official says the major arms control issue facing the two countries is the ``militarization of space.'' It is possible the Kremlin does not want to detract from message by spotlighting the second Strategic Arms Limitation Treaty (SALT II) as a major issue.

Some diplomats theorize that the Soviets also do not want to be seen pressuring the Reagan administration on the issue, for fear that will backfire and generate congressional support for the president.

Another explanation for the low-key treatment is that the Kremlin believes the decision has already been made, and that it is pointless to campaign for continued adherence to the treaty.

Tass, the Soviet news agency, said in a commentary that if the White House has to choose between treaty adherence or an arms buildup, it will chose the latter. The Reagan administration adhered to the treaty, said Tass, only ``until the Pentagon's military programs entered into contradiction with it.''

If the administration should choose to stop abiding by the treaty, some Western analysts expect a major propaganda campaign by the Soviets.

At the same time, the Soviets will likely protest their own innocence of violations. Officials here deny the Soviet Union has taken any steps to undermine SALT II. Nor, they say, does it have any plans to do so -- unless the Reagan administration does first.

The treaty was never formally ratified by the United States. President Carter withdrew it from the Senate when the Soviet Union invaded Afghanistan in 1979.

Both the US and the Soviet Union say they have abided by the provisions of the treaty, notably its 1,200 nuclear warhead limit. But the White House has acknowledged that the US will go over that limit later this year, when the Alaska, a new 24-missile Trident submarine, is launched this summer.

The US is considering removing an older Poseidon submarine from service in order to stay within the limit. Or the president could renounce the treaty entirely, charging that the Soviets have not been abiding by it.

The Soviets will doubtless be eyeing Western European reaction to the White House decision. Says a Western diplomat, ``They'll wait to see how it plays in . . . Bonn, London, and Paris.''